Recently, much progress has been made in increasing the brightness of light emitting device such as light-emitting diodes (LEDs). As a result, it is anticipated that in the years to come, LEDs will become sufficiently bright and inexpensive to serve as a light source in, for example, lamps with adjustable colour, direct view Liquid Crystal Displays (LCDs) and in front and rear projection displays.
Often it is required that a high lumen efficacy is obtained. If LED dies that emit blue light and converting part of the blue light into yellow light are used, a high lumen efficacy may be obtained. Such devices include those where a LED die is covered by a layer of phosphor. However, a drawback with such phosphor covered LED dies is that a lot of the light that is generated exits the phosphor layer in a wrong direction and falls onto the blue die and is absorbed. Because the blue light has to travel through the phosphor layer, while the yellow light is emitted by the phosphor, the blue and yellow light have a different angular distribution. This may result in an undesired discoloration of the light beam in the far field.
Other known solutions include those where place a phosphor layer is arranged relatively far away from a LED and covering a relatively large area. Although the probability may be small that light is emitted back towards the LED, a large phosphor layer is typically needed in such solutions. Another drawback of such devices is that they typically have a large etendue. In fact, for applications such as spotlights, projection displays, side-lit LCDs, it is important to keep the etendue as small as possible. Although, some degree of collimation can be regained by adding, e.g., a so-called Brightness enhancement foil (BEF), such solutions will reflect some of the light in the direction of the source and hence they are not very efficient.
An example of a prior art device is disclosed in European patent application published as EP 1566848. In EP 1566848 a light-emitting device is described, which comprises a LED that is covered by two phosphor layers and where a dichroic filter is arranged between the two-phosphor layers for reducing the amount of backscattered light into the LED.